Shutter Priority Mode is for the times when shutter speed is the primary consideration. Sporting events, that fidgety nephew of yours, birds in flight, etc. Or what if you want to do something a little artsy with your photography. You want to photograph your daughter’s ballet performance but you want some of the pictures to capture the essence of motion, not just freeze her in place. Or what if you want to get that smooth, creamy look when photographing a stream or waterfall. If you tried to do these things in Auto Mode, your camera would think it knows best. It would probably give you a perfect exposure. And a perfectly common, boring photograph.
Just as Aperture Priority Mode allows you to control the Aperture, Shutter Priority Mode allows you to control the shutter speed, and your camera will select the appropriate aperture to create a good exposure.